The term Radio Frequency Identification (RFID) is typically used to refer to a technology that uses radio waves to transfer data from an electronic tag, commonly referred to as RFID tag or transponder, to a reader for the purpose of identifying and tracking an object to which the RFID tag is affixed. The RFID tag's information is stored electronically and the RFID tag includes a small RF transmitter and receiver. An RFID reader transmits an encoded radio signal to interrogate the RFID tag. The RFID tag receives the message and responds with its identification information. A class of RFID tag referred to as passive RFID tags does not use a battery. Instead, a passive RFID tag harvests energy from the transmitted RF interrogation signal and uses the harvested energy to power its electronics. RFID tags that include batteries are typically referred to as active RFID tags.
A number of organizations have established standards for RFID systems including the UHF Class 1 Gen 2 Standard developed by EPCglobal Inc., which is commonly referred to as the EPCglobal Gen 2 standard and defines the physical and logical requirements for a passive-backscatter, interrogator-talks-first (ITF), radio-frequency identification (RFID) system operating in the 860 MHz-960 MHz frequency range. The EPCglobal Gen 2 standard specifies that RFID tag backscatter can use Amplitude Shift Keying (ASK) and/or Phase Shift Keying (PSK) modulation. In addition, the standard requires RFID tags to encode data using FM0 (bi-phase space) baseband or Miller modulation of a subcarrier at the data rate. Under the standard, the reader specifies the encoding to be used by the RFID tag.
FM0 encoding inverts the baseband phase at every symbol boundary. A data 0 has an additional mid-symbol phase inversion. The EPCglobal Gen 2 standard specifies that FM0 signaling shall begin with one of a standard or an extended preamble (specified by the reader). The preambles differ in that the extended preamble (TRext=1) includes the six preamble symbols of the standard preamble (TRext=0) with a pilot tone of 12 leading zeros.
Baseband Miller encoding inverts its phase between two data-0s in sequence. Baseband Miller encoding also places a phase inversion in the middle of a data-1 symbol. The Miller-modulated subcarrier sequences contain two, four, or eight subcarrier cycles per bit, depending on the M value specified by the reader. As with FM0 signaling, the EPCglobal Gen 2 standard specifies that Miller-modulated subcarrier signaling begin with either a standard or an extended preamble. The standard preamble includes a pilot tone having a duration of 4 symbol periods and the extended preamble includes a pilot tone with an additional duration of 12 symbol periods.